Color image forming systems for color image forming apparatuses are roughly classified into four types of transfer drum system, intermediate transfer system, image-on-image system and tandem system.
The transfer drum system is a system in which a transfer sheet wound around a transfer drum composed of a dielectric film is disposed opposite to a photosensitive member, and toners of yellow (Y), magenta (M), cyan (C) and black (Bk) are superimposed on the transfer sheet in sequence by repeating an electrophotographic process consisting of formation, development and transferring of an electrostatic latent image with respect to each of the colors, thereby obtaining a full color image.
The intermediate transfer system is a system in which a full color image is formed by transferring toner images of colors in sequence onto a drum or a belt called an intermediate transfer member in place of the transfer sheet used in the transfer drum system, and then, the full color image is transferred again onto a transfer sheet by one operation.
The image-on-image system is a system in which an image is transferred onto a transfer sheet by repeating an electrophotographic process consisting of formation, development and transferring of an electrostatic latent image with respect to each of yellow (Y), magenta (M), cyan (C) and black (Bk) directly on a photosensitive member.
The tandem system is a system in which a full color image is obtained on a transfer sheet by transferring images formed by image forming units in sequence onto a single piece of transfer sheet fed by a transfer belt.
The color image forming apparatuses using the above-described systems have both merits and disadvantages, respectively. Among them, the color image forming apparatus of the tandem system has excellent characteristics. Because, for example, it can use a large variety of transfer sheets, obtain a full color image of a high quality, and obtain a full color image at a high speed. In particular, the characteristic that a full color image can be obtained at a high speed is not provided for the color image forming apparatuses of the other systems, and therefore, is peculiar to the color image forming apparatus of the tandem system.
In the image forming apparatus of the tandem system, four image forming units must be arranged along the direction of movement of the transfer belt. In most image forming apparatuses of the tandem system, the upper traveling side of the transfer belt generally extends in a horizontal direction, and the four image forming units are arranged on the transfer belt. However, each of the image forming units is provided with a photosensitive member having means for charging, developing, cleaning and the like there around. If the above-described image forming units are juxtaposed on the transfer belt, the image forming apparatus becomes great in width, thereby requiring a large installation space.
In view of this, Japanese Patent Application Laid-open No. 10-239938 discloses an image forming apparatus in which a transfer belt is erected vertically. Since image forming units are arranged vertically along the transfer belt, it is possible to provide an image forming apparatus which can be reduced in width and can be installed in a small space.
However, it is inevitable that the height of a casing becomes greater in the image forming apparatus in which the transfer belt is vertically erected. Therefore, in an image forming apparatus in which the upper surface of the casing is used as a sheet discharging tray, as implemented in many recent printers, there has arisen a problem of poor workability because the position of the sheet discharging tray is too high.